Silicon is a promising material for high capacity anodes in lithium ion batteries (LIB). When alloyed with lithium, the specific capacity (mAh/g) of silicon is an order of magnitude higher than conventional graphite anode materials. However, silicon exhibits a large volume change (up to 400% expansion and contraction) during lithiation (charging) and delithiation (discharging), respectively. For bulk silicon, this creates structural stress gradients within the silicon and results in fractures and mechanical stress failure (pulverization) thereby decreasing effective electrical contact and lifetime of the silicon anode.
Considerable efforts have been undertaken to overcome this intrinsic issue by controlling the morphology and limiting the size of silicon particles to a size below which silicon is less likely to fracture, approximately 50 nm.
Various attempts to avoid the physical damage caused by silicon's expansion/contraction have included nanoscaled and nanostructured silicon in forms such as thin films; nanowires; nanotubes; nanoparticles; mesoporous materials; and nanocomposites. Most of these approaches do not provide viable, cost effective solutions.
One promising method utilizes Si—MgO composites formed by mechanical alloying/solid phase reaction of SiO2 and magnesium according to the reaction:2Mg(s)+SiO2(s)→2MgO(s)+Si(s)  (Formula I)The MgO matrix has shown to buffer the effects of volumetric changes; however, these composites have relatively low electrical conductivity rendering them poorly effective as anode material.
Sub-micron scale, electrochemically active particles dispersed on conductive substrates and supports have long been used for electrochemical cells including fuel cells and batteries. This support structure is an important component with regard to cell efficiency and lifetime. Valve (or refractory) metals particularly, (specifically: Titanium, Niobium, Tantalum, and their alloys) have been used as substrates for electrochemically active materials for over 70 years in application of chemical processing and cathodic protection. These applications utilize the formation of a passivating oxide film over the exposed valve metal areas, as a means of creating a conductive and electrochemically stable support structure for the active material.
Mg has long been used as a magnesiothermic reducing agent for purification of refractory metals. This process is common in production of high capacity, high surface tantalum powders for capacitor applications occurring via the vapor/solid phase reaction:5Mg(g)+Ta2O5(s)→5MgO(s)+2Ta(s)  (Formula II)The resulting magnesium oxide forms a surface coating over the host Ta particles, and is removed using mineral acids.
In one aspect the present invention provides electrically active electrode material for use with a lithium ion cell, the electrochemically active material electrode material comprising a valve metal substrate material formed of filaments or particles of a valve metal not larger than about 10 microns in cross section, and coated with metallurgically bonded silicon particles.
In a preferred embodiment, the valve metal is selected from the group consisting of tantalum, niobium, an alloy of tantalum, an alloy of niobium, hafnium, titanium and aluminum.
In another preferred embodiment, the valve metal filaments have a thickness of less than about 5-10 microns, and preferably have a thickness below about 1 micron.
In one aspect the silicon coating is comprised of nanoscaled nanoparticles.
In another aspect the silicon particles are coated on the valve metal substrate in a stabilizing MgO matrix.
In still another aspect, electrically active electrode material as above described is formed into an anode.
The present invention also provides a method of forming an electrode substrate useful for forming a lithium ion battery comprising the steps of: (a) providing valve metal substrate material formed of filaments or particles of a valve metal not larger than about 10 microns in cross section; and, (b) coating the valve metal substrate material with metallurgically bonded silicon formed by a magnesiothermic reaction of magnesium with silica and the valve metal.
In one aspect of the method, the magnesiothermic reaction is conducted under vacuum or in an inert gas at elevated temperature, preferably an elevated temperature selected from a group consisting of 800-1200° C., 900-1100° C. and 950-1050° C.
In another aspect of the method, the magnesiothermic reaction is conducted for time selected from 2-10 hours, 4-8 hours and 5-6 hours.
In yet another aspect of the method includes the step of removing at least some of the magnesium oxide following the reaction by acid etching.
In one preferred aspect of the method, the valve metal is selected from the group consisting of tantalum, niobium, an alloy of tantalum, an alloy of niobium, hafnium, titanium and aluminum.
In another preferred aspect of the method, the filaments or fibers have a thickness of less than about 5-10 microns, and preferably a thickness below about 1 micron.
In another aspect of the method, the electrochemically active material comprises silicon nanoparticles.
The present invention also provides a lithium ion battery comprising a case containing an anode and a cathode separated from one another, and an electrolyte, wherein the anode is formed of electrically active electrode material comprising the steps of: (a) providing valve metal substrate material formed of filaments or particles of a valve metal not larger than about 10 microns in cross section; and, (b) coating the valve metal substrate material with metallurgically bonded silicon formed by a magnesiothermic reaction of magnesium with silicon and the valve metal.
In yet another aspect of the cell, the valve metal is selected from the group consisting of tantalum, niobium, an alloy of tantalum, an alloy of niobium, hafnium, titanium and aluminum.